1. Field of the Invention
The present invention relates to a driving circuit of driving a light-emitting device.
2. Description of the Related Art
Semiconductor laser diodes are being widely used, for example, for writing in laser beam printers (LBPs) and for writing to or reading from various recording mediums such as compact disks (CDs) and digital versatile disks (DVDs) because they are small in size and low in power consumption. A semiconductor laser diode emits light when a certain threshold current is exceeded. In recent years, with the remarkable reduction in threshold value and improvement in emission efficiency, it has been possible to obtain by a drive current of several milliamperes the amount of emission of light for which a drive current of several ten milliamperes is required in the conventional semiconductor laser diodes. In LBPs, the number of output sheets per minute depends on the switching speed of a semiconductor laser diode and the resolution of an output image depends on the minimum width of an optical pulse that the semiconductor laser diode can output. In a case where a semiconductor diode requires a drive current of several milliamperes such as that for conventional ones, therefore, the parasitic capacitance of the semiconductor laser diode is charged by causing a current by which the semiconductor does not emit light, i.e., a current lower than the threshold, to flow for the purpose of improving the operating speed and the current pulse rise time.
Japanese Patent Application Laid-Open No. 2000-216486 describes a laser drive circuit which biases the gate of an output transistor during a non-outputting period by a diverted current. Also, Japanese Patent Application Laid-Open No. 2003-198047 describes a laser drive circuit which draws out part of a current from a current source to set the bias current during a non-outputting period to a value in the vicinity of the threshold.
Each of the above-described related laser drive circuits biases in advance the gate of an output transistor during a non-outputting by using a current diverted (a partial current drawn out) from a current source to improve response during an outputting period. Currently, the light-emitting device (semiconductor laser diode) have been developed to have an excellent performance, such that a threshold current is reduced into several milliamperes. Accordingly, it is likely that, even at a minimum control voltage, an excessive drive current larger than a suitable level for a desired light emission quantity would flow thereto.
Moreover, a MOS transistor for supplying the drive current has the Early effect. The drive current therefore has a value higher than the ideal value in the vicinity of its minimum and easily exceeds the threshold.